OTTAWA - The battle over the cost of the Harper government's stealth fighter has escalated into a war of numbers between the Parliamentary Budget Officer and the Defence Department.

Kevin Page today released a rebuttal to senior defence officials, who questioned the math in his latest report, which says the cost of the F-35 Lightning II could reach $30 billion over three decades.

Last week, an assistant deputy minister at the Defence Department said the budget officer made a "mathematical error" in calculating the unit cost of the planes, and that estimates on long-term maintenance were erroneous as well.

The F-35 program in the U.S. has seen huge cost overruns, which Page says will drive up the price tag from an estimated US $75 million to US $148 million for each plane.

The department dismisses the figure, but Page pointed out today the Pentagon's latest estimate is US $151 million and that Washington does not sell aircraft to allies at a price less than what it pays.

The jet-fighter deal is expected to be a big issue in the expected spring election because the Liberals have promised to cancel it.